Casting and moulding are the conventional and widely practised bottom-up approach which involved melting the metal and moulding (Shivkumar et al., JOM 43.1, 1991: 26-32). Machining of metal blocks by cutting and drilling to fabricate desired shape is also practised conventionally but is not cost effective (Masuzawa et al., CIRP Annals-Manufacturing Technology 46.2 (1997): 621-628, 49.2 (2000): 473-488; Dornfeld et al., CIRP Annals-Manufacturing Technology 55.2 (2006): 745-768). Loose sintering is the simplest form of metal powder processing which can only produce porous parts and takes longer time for sintering without compaction, while isostatic pressing can produce relatively large parts of high aspect ratio with superior material properties Govindarajan et al., International journal of mechanical sciences 36.4 (1994): 343-357). High-end equipments are necessary for pressing at a very high pressure as well as extremely high temperature that are truly expensive. In roll compaction or powder rolling, metal powder is pressed between two heavy duty pressure rollers that rotate to form a continuous length of metal strip or sheet; however, does not have the versatility to produce any other shape (U.S. Pat. No. 2,033,240, 1936)). Complex solid metallic parts with intricate geometries and thin walls are mostly made through metal injection moulding (MIM) (German et al., MPIF, Princeton, N.J. (1990); Sundberg et al., Applied physics letters 57.7 (1990): 733-734.; Adv.; Johnson et al., Mater. Processes, 161 (4) (2003), pp. 35-39). Major drawback of such a process is very high volume of shrinkage during sintering owing to relatively high binder content and densification by removal of high porosity. In MIM molding the process is limited only to the thermoplastic polymer matrix which again involves temperature for processing. Spark plasma sintering (SPS) is a unique powder processing technique for producing high quality objects in a single step without use of any binder. Metal powder is sintered under pulsed DC current in sparking condition. However, SPS is only limited to produce simple symmetrical shapes and makes use of expensive pulsed DC generator.
The metal powders are difficult to disperse using only electrosteric stabilization owing to their high density, low charge density (thin oxide coating) and bigger particle size. Most of the existing processes involve viscosity change from a low viscosity to high viscous slurry after casting followed by drying and sintering (Sepulveda, 1997; Andersen et al., 2000; Angel et al., 2004; (Shimizu and Matsuzaki, 2007; Kennedy and Lin, 2011). Thickener is reported to be used as a stabilizer/viscosity-enhancer for uniform metal suspension. After casting, no further modification can be done to the formed shapes. In such process no reprocessing can also be done after that, if required.
Recently, metallic parts are also known to be fabricated by 3D printing/rapid prototyping, as an alternative to casting and machining. These techniques are useful for development of objects from computer aided design (CAD) model through layer-by-layer fabrication. However all these techniques are based on melting and deposition of metal powder or thermoplastic polymer as binder. Fugitive based techniques were commonly used for last two decades to generate highly open porous structures. Process like slurry based casting or 3D printing are limited by high drying time, surface contamination due to oxide formation during processing and significant carbon residue after sintering which makes them brittle.
Several prior U.S. Pat. No. 6,045,748, U.S. Pat. No. 7,491,356, US20100092790, U.S. Pat. No. 5,745,834, U.S. Pat. No. 4,197,118 relate to processes of manufacturing articles of metal powder from particulate material and a polymer/binder though various process steps all of which include steps of heating, melting, cooling, drying, and providing a green body before sintering.
U.S. Pat. No. 4,415,528 relates to the method of forming metal alloy shaped parts from a mix of metals and/or individual compounds along with a binder to form a homogeneous mass which form green bodies. The green bodies are then processed by stripping of the binder and raising the temperature of the stripped body below the sintering temperature of the metals and finally sintering was done.
U.S. Pat. No. 2,939,199 relates to process of manufacturing articles from sinterable materials ceramic powders, metal carbides, or their mixtures, by mixing them with a vehicle comprising a thermosetting moulding material and a plasticizer. Moulding results in green shape which is hardened.
However in the above mentioned prior arts, usually most of the process steps are complex and the difficulty in making homogeneous distribution of metal particles in slurry based casting techniques since settling of the particles persists. Hence there remains a continuing need in the art to develop a simple dough processing technique which would have the ability to produce both dense and porous shape formable dough for desired metal objects in a cost-effective way. Further, the process should offer fabrication of diverse size and shape of the components with tailorable microstructure within a reasonable time frame.